Current insights and future directions of LncRNA Morrbid in disease pathogenesis

Non-coding RNAs have emerged as important regulators of gene expression and contributors to many diseases. LncRNA Morrbid, a long non-coding RNA, has been widely studied in recent years. Current literature reports that lncRNA Morrbid is involved in various diseases such as tumors, cardiovascular diseases, inflammatory diseases and metabolic disorder. However, controversial conclusions exist in current studies. As a potential therapeutic target, it is necessary to comprehensively review the current evidence. In this work, we carefully review the literature on Morrbid and discuss each of the hot topics related to lncRNA Morrbid.


Introduction
LncRNA Morrbid is a long non-coding RNA located on the F1 arm of chromosome 2.It is conserved between species and is known as MIR4435-2 H G, AGD2, LINC00978, MIR4435-1 HG, and lncRNA-AWPPH in humans.In August 2016, Jonathan J and his colleagues published an article in Nature, revealing that Morrbid regulates the proapoptotic gene Bcl2l11 (also known as Bim) in a cis manner, extending the lifespan of short-lived bone marrow-derived innate immune cells: eosinophils, neutrophils, and Ly6C hi monocytes.Consequently, it was named Myeloid RNA Regulator of Bim-Induced Death (shortened to lncRNA Morrbid) and abbreviated as Morrbid.In September of the same year, Nature Reviews Immunology published a commentary highly affirming the significance of this work.Given the revealed pathological mechanisms linking Morrbid to cancer progression, Morrbid has been regarded as an oncogene.Potential therapeutic strategies targeting Morrbid may provide novel approaches for clinicians.Therefore, it is imperative to undertake a comprehensive review of the research about Morrbid.Literature reports that lncRNA Morrbid plays a role in various tumors, cardiovascular diseases, inflammatory diseases, and bone metabolic diseases.In this work, we discuss Morrbid's involvement in each disease (Fig. 1).Fig. 1.Morrbid contributes to the pathological process of multiple systemic tumors, hemooncology, inflammatory diseases, cardiovascular diseases and bone metabolism diseases.
H. Yang et al.

Morrbid is involved in tumor mesenchymal transition
Epithelial-mesenchymal transition (EMT) is closely related to tumor metastasis and invasion; High EMT enhances tumor malignancy.During the process, epithelial cells transform into elongated cells with loss of adhesion molecules and acquire a mesenchymal phenotype [20][21][22].Several studies have shown that Morrbid hyperactivation in tumors upregulates several key transcription factors of EMT and key proteins in EMT-associated signaling pathways.These transcription factors and pathway proteins form positive feedback that enhances the Morrbid-mediated effects of EMT.For example, in hepatocellular carcinoma, over-regulated Morrbid upregulates the classical EMT-associated transcription factors: Slug, Snail, and Twist1, which transcriptionally activate N-cadherin and vimentin while repressing E-cadherin expression [6,[23][24][25][26].Additionally, Morrbid promotes vimentin translation by adsorbing miR-506-3p [27].Morrbid adsorbing miR-506-3p also raises the content of TGFβ1 and ZEB1.TGFβ1 activates the Smad2/3 signaling pathway to upregulate vimentin expression, while ZEB1 binds to the E-box sequence in the E-cadherin promoter, leading to its downregulation, and binds to specific sequences in the vimentin promoter to activate transcription [27][28][29][30].Studies have shown that vimentin can feedback upregulate Snail and Slug [23,[25][26][27]29,30].Furthermore, Morrbid binding with YBX1 stabilizes Snail's mRNA and binds to the promoter of PIK3CA in the nucleus to improve its transcription [3].These mechanisms help enhance the pro-EMT effects of Morrbid in hepatocellular carcinoma.Morrbid's promotion of tumor EMT involves multiple signaling pathways [31][32][33], and the existence of signaling crossover between each pathway makes the mechanism of its promotion of EMT more complex.Lu Pei et al. confirmed that more than 200 proteins interacting with Morrbid were mainly enriched in Hippo, NF-kappa B, Wnt, mTOR, MAPK4, and JAKs pathways by RNA-pulldown experiments on T24 cells [34].Findings from other tumors also suggest that Morrbid promotes activation of classical EMT signaling pathways such as PIK3K/AKT/mTOR, Wnt/β-catenin, JAK/STAT, TGF-β/smads, and Hippo/YAP1.For instance, Morrbid binds to the promoter of PIK3CA in the nucleus to enhance its transcription and interacts with miR-150 to upregulate GRB2-associated binder 1 (GAB1) expression, which increases the phosphorylation of ERK1/2 and AKT in hepatocellular carcinoma [3,34].In breast and lung cancer, Morrbid upregulates β-catenin by promoting its translation and inhibiting its degradation and upregulates frizzled homolog 7 (FZD), both of which are key molecules in the Wnt signaling pathway [35][36][37][38].Several other studies have reported that Morrbid, by upregulating TGF-β and TGF-β receptors in tumors, activates TGF-β/smad to promote EMT [39,40].Furthermore, Morrbid promotes EMT in rectal cancer via Hippo/YAP1 by upregulating YAP1 expression [34,41].Studies show that down-regulation of Morrbid effectively inhibits cancer cell proliferation, survival, and EMT.For example, in gastric cancer cells, after si-Morrbid interference, the expression levels of N-Cadherin, MMP-9, VEGF, and a-SMA are significantly lower than in the sh-NC groups [18,42].It is evident that Morrbid regulates EMT in a variety of tumors, and this regulatory effect is closely linked to its regulation of cell proliferation, migration, and invasion due to shared signaling pathways.Although the mechanism of Morrbid's pro-EMT effect is complex and requires more evidence for EMT effects in tumors, Morrbid remains a candidate target for tumor EMT based on the consistency of its EMT effects on cancer cells [18,[42][43][44][45](Fig. 2).
However, in contrast to most findings, Hongfei Yu et al. found that Morrbid acts as a tumor suppressor in rectal cancer [65].They found that the knockdown of Morrbid in SW480 and RKO cells or Morrbid overexpression in DLD1 and HCT116 cells did not alter the cell biological behaviors.In vivo, they observed that the growth of MC38, a type of rectal cancer cell, was faster in Morrbid − /− mice compared to WT mice.However, the growth of MC38 cells with Morrbid knockout made no difference between Morrbid − /− mice and WT mice.This indicated that the microenvironment of Morrbid − /− mice significantly promoted tumor growth.This conclusion was subsequently verified in a co-culture experiment of MC38 with spleen cells isolated from Morrbid − /− mice and WT mice.They confirmed that the increase in polymorphonuclear myeloid-derived suppressor cells (PMN-MDSC) in the rectal tumor stroma of Morrbid − /− mice might be the mechanism by which Morrbid knockout enhances antitumor immunosuppression and facilitates the progression of rectal cancer.Although this study shows that the role of Morrbid in tumor immunity is still controversial, Morrbid is indeed involved in tumor immunity.

Morrbid alters tumor metabolism
All the literature we consulted showed that several reports provide evidence that Morrbid is connected with tumor metabolism, especially in glucose and lipid metabolism.For instance, Morrbid silence in colon cancer results in GLUT-1 down-regulation [66].In rectal cancer, Morrbid knockdown results in lipid accumulation in neutrophils and PMN-MDSC, which may be related to the enhanced immunosuppressive activity of PMN-MDSC [65].These results imply that Morrbid hyperactivation may positively regulate glucose metabolism to meet the hypermetabolic characteristics of tumor growth [67].In addition, based on the TCGA database, Morrbid, as a lncRNA related to glucose metabolism, is screened out.CeRNA network analysis reveals Morrbid hyperactivation may increase forkhead box protein M1 (FOXM1) expression by Competitively binding hsa-miR-125b-5p [68].FOXM1 is a transcription factor (TF) of the Fox family, associated with insulin, insulin-like growth factor, leptin and adiponectin secretion, and the expression of glucose transporter type 1 (GLUT1) and glucose transporter type 2 (GLUT2) [69].Other research groups reported that IL-6/STAT3, PI3K/Akt, Ras-ERK, YAP1/Akt/GSK3β signaling axis can activate FOXM1 expression [70][71][72], among which STAT3/FOXM1/GLUT1 signaling pathway is closely related to glycolysis [73].Consistent with those results, Morrbid sponges miR-206 and upregulates YAP1, which can activate FOXM1 expression through the Akt/GSK3β signaling pathway in rectal cancer [41].In short, these results indicate that Morrbid is most likely to play a crucial role in tumor glucolipid metabolism.However, the mechanism by which Morrbid regulates tumor glycolipid metabolism needs to be further elucidated (Fig. 3).

Morrbid and hematologic malignancy
Morrbid, enriched in short-lived eosinophils, neutrophils, and Ly6C hi monocytes, causes H3K27me3 deposition on the Bcl2l11 promoter by working with the Polycomb repressive complex 2 (PRC2), leading to Bim downregulation [74].These innate short-lived immune cells play a crucial role in immune response, inflammation, and repair.Leukemia is a highly malignant proliferative disease often accompanied by gene mutations, making it difficult to cure [75][76][77][78].Genetic susceptibility, combined with additional shocks such as infection, inflammation, radiation, and senescence, usually leads to the rapid progression of such diseases.Acute inflammatory stress itself can also cause damage or stemness loss of blood progenitors [79][80][81].Some groups have found that Morrbid participates in hematological tumor progression.
Tet2, a member of the ten-eleven translocation dioxygenases (TETs) family, physiologically catalyzes dynamic DNA demethylation and regulates blood cell lineage determination and differentiation [82,83].Therefore, Tet2 mutations occur in almost all hematologic tumors [84,85].Humans with a single absence of Tet2 function may survive with a preleukemic state because Tet1/3 could partially H. Yang et al. compensate for Tet2 loss.However, LPS can induce Tet2 − /− mice to develop myelodysplastic syndromes (MDS) or chronic myeloid leukemia (CML) [83].The mechanism of LPS-induced MDS or CML may be partially caused by IL-6/Shp2/Stat3 axis over-activation, leading to upregulation of Morrbid [86].Allosteric inhibition of Shp2 (SHP099, a pan-Stat3 inhibitor) or Morrbid knockout can alleviate abnormal clonal hematopoiesis in Tet2 − /− mice treated with LPS [53].Additionally, juvenile myelomonocytic leukemia (JMML) is a hyperinflammatory syndrome, and allogeneic hematopoietic stem cell transplantation is the only effective treatment.35 % of JMML cases are caused by PTPN11 encoding SHP2 mutation.According to a study, Shp2 and Morrbid biallelic knockout can alleviate anemia and reduce inflammatory cells in the bone marrow of mice, with a longer median survival compared to single Shp2 knockout mice [87].A high level of Morrbid in human JMML with PTPN11 mutations reflects a lower overall survival rate [87].
Two other studies focus on the Flt3 mutation associated with AML, which encodes the type III receptor tyrosine kinase [88,89].Flt3-ITD refers to different lengths of tandem duplications inserted into the Flt3 near the membrane structure domain [90,91].FLT3-ITD activates multiple signaling pathways downstream of FLT3, such as RAS/MAPK, PI3K/AKT, and JAK/STAT, leading to blood cell proliferation, survival, and inhibition of apoptosis [88,89,92].Zhigang Cai and his colleagues found that Morrbid hyperactivation occurs in Tet2 − /− ; Flt3-ITD mice.Morrbid knockout significantly reduced the percentage of leukemia cells in the bone marrow and peripheral blood and alleviated extramedullary infiltration in mice.Additionally, Morrbid interacts with Rho-associated protein kinase 2 (ROCK2) to promote abnormal T lymphocyte proliferation and inhibit its apoptosis [93].These studies indicate that Morrbid may be a new target for hematological malignancy treatment in synergy with other drugs [78].

Morrbid and inflammatory diseases
Based on the findings of overexpression of Morrbid in various tumors, it has been shown that aberrant activation of Morrbid is closely related to immune cells and inflammation.The IL-6/Shp2/Stat3 axis upregulates Morrbid, which subsequently downregulates Bim to prolong the lifespan of intrinsic immune cells [74], favoring the expansion of the immune range and enhancement of immune competence [94,95].This observation suggests that Morrbid regulation of immunity is a double-edged sword in different disease states.For example, the increased immune activity of dendritic cells (DCs) in individuals with spontaneous highly antiretroviral replication may be due to the upregulation of mTORC1 by Morrbid hyperactivation through H3K27ac enrichment at the RPTOR promoter, resulting in enhanced beta-oxidation and oxidative phosphorylation [96].Morrbid knockout causes mice to be highly susceptible to L. monocytogenes infection [74].Additionally, acute and chronic lymphocytic choriomeningitis virus (LCMV) infection induces CD8 + T cell activation and significant upregulation of Morrbid.Morrbid knockdown results in a significantly higher number of CD8 + T cells in the peripheral circulation compared to wild-type (WT) mice after LCMV infection.The proportions of IFN-γ, TNF-α, and granzyme B (GzmB) T cells, as well as the subpopulation of memory T cells, are significantly increased [97].This suggests that Morrbid restricts CD8 + T cell numbers under physiological conditions and is associated with maintaining CD8 + T cell function homeostasis.
In conclusion, Morrbid is required to maintain an appropriate inflammatory and cellular immune response.Dysregulation of Morrbid expression may be one of the pathological mechanisms involved in the development and progression of autoimmune diseases, but the molecular mechanisms of Morrbid involvement in autoimmune diseases need to be further elucidated.

Morrbid and cardiovascular diseases
Cardiovascular diseases (CVD) have become the leading cause of death worldwide [104,105].As early as 2016, Morrbid was identified as a lncRNA associated with myocardial hypertrophy [106].Although the mechanism by which Morrbid participates in cardiac hypertrophy remains unclear, studies have reported that Morrbid is significantly downregulated in transverse aortic constriction (TAC) mice with adenosine receptor A2A overexpression, suggesting that Morrbid may mediate the cardioprotective effect of A2A or directly participate in the pathological process of cardiac hypertrophy [106].Clinical data also show that Morrbid levels rise significantly in the peripheral blood of patients with coronary artery diseases and in the carotid atherosclerotic plaques of patients with type 2 diabetes mellitus.Multivariate logistic regression analysis indicates that Morrbid level in serum is an independent risk factor for coronary artery disease diagnosis and prognosis, and statins can reduce Morrbid levels in serum [107].A recent study has reported that Morrbid acts as a modulator of monocyte-macrophage phenotypes, which are involved in atherogenesis, suggesting that Morrbid is a potential diagnostic and prognostic index for atherosclerosis [108].This suggesting that Morrbid is a potential diagnostic and prognostic index for atherosclerosis.
Additionally, Morrbid has been found to be involved in the pathology of ischemic heart injury.In mice with 24-h infarction, Morrbid was significantly upregulated and exerted a protective effect on the heart through Serpine1 [109], a molecule associated with blood vessels and coagulation.The cardioprotective effect of Morrbid targeting Serpine1 may be related to improved microcirculation.Another study showed that Morrbid was upregulated in the plasma of reperfused patients and in the cardiac tissues of I/R (30 min of ischemia followed by 2 h of reperfusion) mice.In this context, upregulated Morrbid targeted mitochondrial fission protein 1 (MTFP1) in cardiomyocytes via MiR-125a-5p, promoting cardiac injury during I/R [109][110][111][112][113][114][115][116][117][118].These results suggest that Morrbid may target different molecules with different functions in various heart disease models.
H. Yang et al.

Morrbid and metabolic diseases
As stated earlier, Morrbid regulates glucolipid metabolism in tumor cells and immune cells, so redundant details will be omitted here.This section introduces Morrbid's role in bone metabolic disease pathology.Several pieces of evidence have shown that Morrbid is necessary for physiological bone metabolism, and its down-regulation promotes the development of bone metabolic diseases [119,120].Morrbid down-regulation in the serum of osteoporosis patients leads to an imbalance in the ratio of type I collagen a1 and α2 in osteoblasts and bone microstructural abnormalities [121,122].BMP-2, an osteoblast differentiation inducer, induces Morrbid and runt-related transcription factor 2 (RUNX2) expression in human bone marrow mesenchymal stem cells (hMSC-BM).RUNX2, a fundamental transcription factor for bone metabolism and development, works with Morrbid to promote chondrocyte proliferation and anti-apoptotic effects [123].Runx2 itself promotes chondrocyte differentiation into osteoblasts [123,124].RUNX2 itself promotes the differentiation of chondrocytes into osteoblasts.Additionally, Morrbid overexpression promotes chondrocyte survival in osteoarthritis by targeting miR-510-3p/IL-17A expression [125].In summary, these findings indicate that Morrbid reduction is harmful to the maintenance of bone microstructure, while Morrbid overexpression promotes chondrogenic differentiation and osteogenesis.

Discussion
As early as 2015, Morrbid was reported to be involved in the pathological processes of lung cancer and cardiac hypertrophy [106,126].Morrbid became famous in 2016 for its involvement in regulating the life span of circulating short-lived cells [127].Subsequently, numerous studies have been conducted on various tumors, cardiovascular diseases, inflammatory diseases, and bone metabolism.
Oncological research has revealed that Morrbid is over-activated in human tumor tissues, with its serum levels positively correlated with tumor malignancy and poor prognosis.Mechanistic studies have shown that Morrbid overexpression promotes cancer cell proliferation, migration, epithelial-mesenchymal transition (EMT), immune escape, and drug resistance [43,128,129].Thus, Morrbid serum levels could serve as an indicator for clinical cancer diagnosis and prognosis, and targeting Morrbid or using it in combination with antitumor therapies offers new insights for clinicians.
Furthermore, studies on hematologic neoplasms and inflammatory diseases have shown that physiological levels of Morrbid are essential for a normal immune response and for limiting excessive inflammatory responses.Therefore, one of the challenges in targeting Morrbid for antineoplastic drug therapy is the potential risk of inducing excessive inflammatory responses or immunosuppression.Additionally, Morrbid serum levels may also serve as indicators for the development and progression of atherosclerosisrelated diseases and the efficacy of statin therapy.
However, in a study on rectal cancer by Hongfei Yu and colleagues, it was found that Morrbid was localized in the mesenchyme of rectal cancer rather than in the cancer cells themselves [65].Knocking down or overexpressing Morrbid in rectal cancer cells did not affect their biological behavior.Contrary to most tumor studies, this study suggests that Morrbid exerts an inhibitory effect, and Morrbid deficiency actually promotes tumor progression [65].The authors discussed that the inconsistent results might be due to other studies not accounting for lncRNA CYTOR, a highly homologous lncRNA to human Morrbid, which is also highly expressed in tumor tissues and promotes tumor progression [65,[130][131][132][133][134][135].Searches in the NCBI and Ensembl databases confirmed sequence homology between human lncRNA CYTOR and Morrbid.The NCBI database even lists Cytor as one of the Morrbid aliases in mice, suggesting shared functions between the two.Therefore, species and primer specificity should be considered when designing primers for Morrbid.However, there are controversial points in Yu's study [65].For instance, the expected efficiency of RNAi interference with low Morrbid expression in rectal cancer cells may not have been achieved.Additionally, the overexpression levels of Morrbid might have surpassed physiological conditions, potentially activating certain signaling pathways and altering cellular behavior.Even if Morrbid is localized in the interstitial cells of cancer tissue, it could affect rectal cancer cells through intercellular communication, such as via exosomes.
In summary, most Morrbid research has focused on tumors.However, it is evident that more work is needed to fully understand Morrbid's role in metabolism regulation, inflammation modulation, immune regulation, and involvement in autoimmune diseases.This underscores the urgency and importance of further research in these areas and the potential for significant contributions to our understanding of Morrbid's functions and its implications for disease.

Declaration of competing interest
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:Chunxiang Zhang reports financial support was provided by National Natural Science Foundation of China, China.If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
H. Yang et al.

Fig. 3 .
Fig. 3. Morrbid may regulate metabolism by activating a key transcription factor FOXM1 in tumors.